Process that results in the interconversion of chemical species
Athermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.
Achemical reaction is a process that leads to thechemical transformation of one set ofchemical substances to another.[1] When chemical reactions occur, theatoms are rearranged and the reaction is accompanied by anenergy change as new products are generated. Classically, chemical reactions encompass changes that only involve the positions ofelectrons in the forming and breaking ofchemical bonds betweenatoms, with no change to thenuclei (no change to the elements present), and can often be described by achemical equation.Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions ofunstable andradioactiveelements where both electronic and nuclear changes can occur.
The substance (or substances) initially involved in a chemical reaction are calledreactants or reagents. Chemical reactions are usually characterized by achemical change, and they yield one or moreproducts, which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-calledelementary reactions, and the information on the precise course of action is part of thereaction mechanism. Chemical reactions are described withchemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions.
Chemical reactions happen at a characteristicreaction rate at a given temperature and chemical concentration. Some reactions produceheat and are calledexothermic reactions, while others may require heat to enable the reaction to occur, which are calledendothermic reactions. Typically, reaction rates increase with increasing temperature because there is morethermal energy available to reach the activation energy necessary for breaking bonds between atoms.
A reaction may be classified asredox in whichoxidation andreduction occur or non-redox in which there is no oxidation and reduction occurring. Most simple redox reactions may be classified as a combination, decomposition, or single displacement reaction.
Different chemical reactions are used duringchemical synthesis in order to obtain the desired product. Inbiochemistry, a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) formmetabolic pathways. These reactions are oftencatalyzed by proteinenzymes. Enzymes increase the rates of biochemical reactions, so thatmetabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperature and concentrations present within acell.
Antoine Lavoisier developed the theory of combustion as a chemical reaction with oxygen.
Chemical reactions such as combustion in fire,fermentation and the reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as theFour-Element Theory ofEmpedocles stating that any substance is composed of the four basic elements – fire, water, air and earth. In theMiddle Ages, chemical transformations were studied byalchemists. They attempted, in particular, to convertlead intogold, for which purpose they used reactions of lead and lead-copper alloys withsulfur.[2]
From the 16th century, researchers includingJan Baptist van Helmont,Robert Boyle, andIsaac Newton tried to establish theories of experimentally observed chemical transformations. Thephlogiston theory was proposed in 1667 byJohann Joachim Becher. It postulated the existence of a fire-like element called "phlogiston", which was contained within combustible bodies and released duringcombustion. This proved to be false in 1785 byAntoine Lavoisier who found the correct explanation of the combustion as a reaction with oxygen from the air.[8]
Regarding theorganic chemistry, it was long believed that compounds obtained from living organisms were too complex to be obtainedsynthetically. According to the concept ofvitalism, organic matter was endowed with a "vital force" and distinguished from inorganic materials. This separation was ended however by the synthesis ofurea from inorganic precursors byFriedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry includeAlexander William Williamson with hissynthesis ofethers andChristopher Kelk Ingold, who, among many discoveries, established the mechanisms ofsubstitution reactions.
As seen from the equationCH4 + 2O2 → CO2 + 2 H2O, a coefficient of 2 must be placed before theoxygen gas on the reactants side and before thewater on the products side so that, as per the law of conservation of mass, the quantity of each element does not change during the reaction.
Chemical equations are used to graphically illustrate chemical reactions. They consist ofchemical orstructural formulas of the reactants on the left and those of the products on the right. They are separated by an arrow (→) which indicates the direction and type of the reaction; the arrow is read as the word "yields".[10] The tip of the arrow points in the direction in which the reaction proceeds. A double arrow (⇌) pointing in opposite directions is used forequilibrium reactions. Equations should be balanced according to thestoichiometry, the number of atoms of each species should be the same on both sides of the equation. This is achieved by scaling the number of involved molecules (A, B, C and D in a schematic example below) by the appropriate integersa, b, c andd.[11]
a A +b B →c C +d D
More elaborate reactions are represented by reaction schemes, which in addition to starting materials and products show important intermediates ortransition states. Also, some relatively minor additions to the reaction can be indicated above the reaction arrow; examples of such additions are water, heat, illumination, acatalyst, etc. Similarly, some minor products can be placed below the arrow, often with a minus sign.
Retrosynthetic analysis can be applied to design a complex synthesis reaction. Here the analysis starts from the products, for example by splitting selected chemical bonds, to arrive at plausible initial reagents. A special arrow (⇒) is used in retro reactions.[12]
Elementary reactions
Theelementary reaction is the smallest division into which a chemical reaction can be decomposed, it has no intermediate products.[13] Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially. The actual sequence of the individual elementary reactions is known asreaction mechanism. An elementary reaction involves a few molecules, usually one or two, because of the low probability for several molecules to meet at a certain time.[14]
Isomerization ofazobenzene, induced by light (hν) or heat (Δ)
The most important elementary reactions are unimolecular and bimolecular reactions. Only one molecule is involved in a unimolecular reaction; it is transformed by isomerization or adissociation into one or more other molecules. Such reactions require the addition of energy in the form of heat or light. A typical example of a unimolecular reaction is thecis–transisomerization, in which the cis-form of a compound converts to the trans-form or vice versa.[15]
In a typicaldissociation reaction, a bond in a molecule splits (ruptures) resulting in two molecular fragments. The splitting can behomolytic orheterolytic. In the first case, the bond is divided so that each product retains an electron and becomes a neutralradical. In the second case, both electrons of the chemical bond remain with one of the products, resulting in chargedions. Dissociation plays an important role in triggeringchain reactions, such ashydrogen–oxygen orpolymerization reactions.
Dissociation of a molecule AB into fragments A and B
Another possibility is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, inredox and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton. This type of reaction is also calledmetathesis.
Most chemical reactions are reversible; that is, they can and do run in both directions. The forward and reverse reactions are competing with each other and differ inreaction rates. These rates depend on the concentration and therefore change with the time of the reaction: the reverse rate gradually increases and becomes equal to the rate of the forward reaction, establishing the so-called chemical equilibrium. The time to reach equilibrium depends on parameters such as temperature, pressure, and the materials involved, and is determined by theminimum free energy. In equilibrium, theGibbs free energy of reaction must be zero. The pressure dependence can be explained with theLe Chatelier's principle. For example, an increase in pressure due to decreasing volume causes the reaction to shift to the side with fewer moles of gas.[16]
The reaction yield stabilizes at equilibrium but can be increased by removing the product from the reaction mixture or changed by increasing the temperature or pressure. A change in the concentrations of the reactants does not affect the equilibrium constant but does affect the equilibrium position.
Thermodynamics
Chemical reactions are determined by the laws ofthermodynamics. Reactions can proceed by themselves if they areexergonic, that is if they release free energy. The associated free energy change of the reaction is composed of the changes of two different thermodynamic quantities,enthalpy andentropy:[17]
.
G: free energy,H: enthalpy,T: temperature,S: entropy,Δ: difference (change between original and product)
Reactions can beexothermic, where ΔH is negative and energy is released. Typical examples of exothermic reactions arecombustion,precipitation andcrystallization, in which ordered solids are formed from disordered gaseous or liquid phases. In contrast, inendothermic reactions, heat is consumed from the environment. This can occur by increasing the entropy of the system, often through the formation of gaseous or dissolved reaction products, which have higher entropy. Since the entropy term in the free-energy change increases with temperature, many endothermic reactions preferably take place at high temperatures. On the contrary, many exothermic reactions such as crystallization occur preferably at lower temperatures. A change in temperature can sometimes reverse the sign of the enthalpy of a reaction, as for thecarbon monoxide reduction ofmolybdenum dioxide:
;
This reaction to formcarbon dioxide andmolybdenum is endothermic at low temperatures, becoming less so with increasing temperature.[18] ΔH° is zero at1855 K, and the reaction becomes exothermic above that temperature.
Changes in temperature can also reverse the direction tendency of a reaction. For example, thewater gas shift reaction
is favored by low temperatures, but its reverse is favored by high temperatures. The shift in reaction direction tendency occurs at1100 K.[18]
Reactions can also be characterized by theirinternal energy change, which takes into account changes in the entropy, volume andchemical potentials. The latter depends, among other things, on theactivities of the involved substances.[19]
U: internal energy,S: entropy,p: pressure,μ: chemical potential,n: number of molecules,d:small change sign
Kinetics
The speed at which reactions take place is studied byreaction kinetics. The rate depends on various parameters, such as:
Reactant concentrations, which usually make the reaction happen at a faster rate if raised through increased collisions per unit of time. Some reactions, however, have rates that areindependent of reactant concentrations, due to a limited number of catalytic sites. These are calledzero order reactions.
Surface area available for contact between the reactants, in particular solid ones in heterogeneous systems. Larger surface areas lead to higher reaction rates.
Pressure – increasing the pressure decreases the volume between molecules and therefore increases the frequency of collisions between the molecules.
Activation energy, which is defined as the amount of energy required to make the reaction start and carry on spontaneously. Higher activation energy implies that the reactants need more energy to start than a reaction with lower activation energy.
Temperature, which hastens reactions if raised, since higher temperature increases the energy of the molecules, creating more collisions per unit of time,
The presence or absence of acatalyst. Catalysts are substances that make weak bonds with reactants or intermediates and change the pathway (mechanism) of a reaction which in turn increases the speed of a reaction by lowering theactivation energy needed for the reaction to take place. A catalyst is not destroyed or changed during a reaction, so it can be used again.
For some reactions, the presence ofelectromagnetic radiation, most notablyultraviolet light, is needed to promote the breaking of bonds to start the reaction. This is particularly true for reactions involvingradicals.
Several theories allow calculating the reaction rates at the molecular level. This field is referred to asreaction dynamics. The ratev of afirst-order reaction, which could be the disintegration of a substance A, is given by:
Its integration yields:
Herek is the first-order rate constant, having dimension 1/time, [A](t) is the concentration at a timet and [A]0 is the initial concentration. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself can be described with a characteristichalf-life. More than one time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows theArrhenius equation:
In a synthesis reaction, two or more simple substances combine to form a more complex substance. These reactions are in the general form:
Two or more reactants yielding one product is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination ofiron andsulfur to formiron(II) sulfide:
Another example is simple hydrogen gas combined with simple oxygen gas to produce a more complex substance, such as water.[21]
A decomposition reaction is when a more complex substance breaks down into its more simple parts. It is thus the opposite of a synthesis reaction and can be written as[21]
In asingle displacement reaction, a single uncombined element replaces another in a compound; in other words, one element trades places with another element in a compound.[21] These reactions come in the general form of:
One example of a single displacement reaction is whenmagnesium replaces hydrogen in water to make solidmagnesium hydroxide and hydrogen gas:
Double displacement
In adouble displacement reaction, the anions and cations of two compounds switch places and form two entirely different compounds. These reactions are in the general form:[21]
For example, whenbarium chloride (BaCl2) andmagnesium sulfate (MgSO4) react, the SO42− anion switches places with the 2Cl− anion, giving the compounds BaSO4 and MgCl2.
Reactions that proceed in the forward direction (from left to right) to approach equilibrium are often calledspontaneous reactions, that is, is negative, which means that if they occur at constant temperature and pressure, they decrease theGibbs free energy of the reaction. They require less energy to proceed in the forward direction.[23] Reactions are usually written as forward reactions in the direction in which they are spontaneous. Examples:
Reactions that proceed in the backward direction to approach equilibrium are often callednon-spontaneous reactions, that is, is positive, which means that if they occur at constant temperature and pressure, they increase theGibbs free energy of the reaction. They require input of energy to proceed in the forward direction.[23][24] Examples include:
Charging a normal DC battery (consisting ofelectrolytic cells) from an external electrical power source[25]
CO2carbon dioxide +H2O water +photonslight energy →[CH2O]carbohydrate +O2 oxygen
Combustion
In acombustion reaction, an element or compound reacts with an oxidant, usuallyoxygen, often producing energy in the form ofheat orlight. Combustion reactions frequently involve ahydrocarbon. For instance, the combustion of 1 mole (114 g) of octane in oxygen
releases 5500 kJ. A combustion reaction can also result fromcarbon,magnesium orsulfur reacting with oxygen.[27]
Oxidation and reduction
Illustration of a redox reactionSodium chloride is formed through the redox reaction of sodium metal and chlorine gas
Redox reactions can be understood in terms of the transfer of electrons from one involved species (reducing agent) to another (oxidizing agent). In this process, the former species isoxidized and the latter isreduced. Though sufficient for many purposes, these descriptions are not precisely correct. Oxidation is better defined as an increase inoxidation state of atoms and reduction as a decrease in oxidation state. In practice, the transfer of electrons will always change the oxidation state, but there are many reactions that are classed as "redox" even though no electron transfer occurs (such as those involvingcovalent bonds).[28][29]
In the following redox reaction, hazardoussodium metal reacts with toxicchlorine gas to form the ionic compoundsodium chloride, or common table salt:
In the reaction, sodium metal goes from an oxidation state of 0 (a pure element) to +1: in other words, the sodium lost one electron and is said to have been oxidized. On the other hand, the chlorine gas goes from an oxidation of 0 (also a pure element) to −1: the chlorine gains one electron and is said to have been reduced. Because the chlorine is the one reduced, it is considered the electron acceptor, or in other words, induces oxidation in the sodium – thus the chlorine gas is considered the oxidizing agent. Conversely, the sodium is oxidized or is the electron donor, and thus induces a reduction in the other species and is considered thereducing agent.
Which of the involved reactants would be a reducing or oxidizing agent can be predicted from theelectronegativity of their elements. Elements with low electronegativities, such as most metals, easily donate electrons and oxidize – they are reducing agents. On the contrary, many oxides or ions with high oxidation numbers of their non-oxygen atoms, such asH 2O 2,MnO− 4,CrO 3,Cr 2O2− 7, orOsO 4, can gain one or two extra electrons and are strong oxidizing agents.
For somemain-group elements the number of electrons donated or accepted in a redox reaction can be predicted from theelectron configuration of the reactant element. Elements try to reach the low-energynoble gas configuration, and thereforealkali metals andhalogens will donate and accept one electron, respectively. Noble gases themselves are chemically inactive.[30]
The overall redox reactioncan be balanced by combining the oxidation and reduction half-reactions multiplied by coefficients such that the number of electrons lost in the oxidation equals the number of electrons gained in the reduction.
An important class of redox reactions are the electrolyticelectrochemical reactions, where electrons from the power supply at the negative electrode are used as the reducing agent and electron withdrawal at the positive electrode as the oxidizing agent. These reactions are particularly important for the production of chemical elements, such aschlorine[31] oraluminium. The reverse process, in which electrons are released in redox reactions andchemical energy is converted toelectrical energy, is possible and used inbatteries.
In complexation reactions, severalligands react with a metal atom to form acoordination complex. This is achieved by providinglone pairs of the ligand into emptyorbitals of the metal atom and formingdipolar bonds. The ligands areLewis bases, they can be both ions and neutral molecules, such as carbon monoxide, ammonia or water. The number of ligands that react with a central metal atom can be found using the18-electron rule, saying that thevalence shells of atransition metal will collectively accommodate 18electrons, whereas the symmetry of the resulting complex can be predicted with thecrystal field theory andligand field theory. Complexation reactions also includeligand exchange, in which one or more ligands are replaced by another, and redox processes which change the oxidation state of the central metal atom.[32]
Acid–base reactions
In theBrønsted–Lowry acid–base theory, anacid–base reaction involves a transfer ofprotons (H+) from one species (theacid) to another (thebase). When a proton is removed from an acid, the resulting species is termed that acid'sconjugate base. When the proton is accepted by a base, the resulting species is termed that base'sconjugate acid.[33] In other words, acids act as proton donors and bases act as proton acceptors according to the following equation:
The reverse reaction is possible, and thus the acid/base and conjugated base/acid are always in equilibrium. The equilibrium is determined by theacid and base dissociation constants (Ka andKb) of the involved substances. A special case of the acid-base reaction is theneutralization where an acid and a base, taken at the exact same amounts, form a neutralsalt.
Acid-base reactions can have different definitions depending on the acid-base concept employed. Some of the most common are:
Arrhenius definition: Acids dissociate in water releasing H3O+ ions; bases dissociate in water releasing OH− ions.
Brønsted–Lowry definition: Acids are proton (H+) donors, bases are proton acceptors; this includes the Arrhenius definition.
Lewis definition: Acids are electron-pair acceptors, and bases are electron-pair donors; this includes the Brønsted-Lowry definition.
Precipitation
Precipitation
Precipitation is the formation of a solid in a solution or inside another solid during a chemical reaction. It usually takes place when the concentration of dissolved ions exceeds thesolubility limit[34] and forms an insoluble salt. This process can be assisted by adding a precipitating agent or by the removal of the solvent. Rapid precipitation results in anamorphous or microcrystalline residue and a slow process can yield singlecrystals. The latter can also be obtained byrecrystallization from microcrystalline salts.[35]
Solid-state reactions
Reactions can take place between two solids. However, because of the relatively smalldiffusion rates in solids, the corresponding chemical reactions are very slow in comparison to liquid and gas phase reactions. They are accelerated by increasing the reaction temperature and finely dividing the reactant to increase the contacting surface area.[36]
Reactions at the solid/gas interface
The reaction can take place at the solid|gas interface, surfaces at very low pressure such asultra-high vacuum. Viascanning tunneling microscopy, it is possible to observe reactions at the solid|gas interface in real space, if the time scale of the reaction is in the correct range.[37][38] Reactions at the solid|gas interface are in some cases related to catalysis.
Many important processes involve photochemistry. The premier example isphotosynthesis, in which most plants usesolar energy to convertcarbon dioxide and water intoglucose, disposing ofoxygen as a side-product. Humans rely on photochemistry for the formation of vitamin D, andvision is initiated by a photochemical reaction ofrhodopsin.[15] Infireflies, anenzyme in the abdomen catalyzes a reaction that results inbioluminescence.[40] Many significant photochemical reactions, such as ozone formation, occur in the Earth atmosphere and constituteatmospheric chemistry.
Schematic potential energy diagram showing the effect of a catalyst in an endothermic chemical reaction. The presence of a catalyst opens a different reaction pathway (in red) with lower activation energy. The final result and the overall thermodynamics are the same.Solid heterogeneous catalysts are plated on meshes in ceramiccatalytic converters in order to maximize their surface area. This exhaust converter is from aPeugeot 106 S2 1100
Incatalysis, the reaction does not proceed directly, but through a reaction with a third substance known ascatalyst. Although the catalyst takes part in the reaction, forming weak bonds with reactants or intermediates, it is returned to its original state by the end of the reaction and so is not consumed. However, it can be inhibited, deactivated or destroyed by secondary processes. Catalysts can be used in a different phase (heterogeneous) or in the same phase (homogeneous) as the reactants. In heterogeneous catalysis, typical secondary processes includecoking where the catalyst becomes covered bypolymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid-liquid system or evaporate in a solid–gas system. Catalysts can only speed up the reaction – chemicals that slow down the reaction are called inhibitors.[41][42] Substances that increase the activity of catalysts are called promoters, and substances that deactivate catalysts are called catalytic poisons. With a catalyst, a reaction that is kinetically inhibited by high activation energy can take place in the circumvention of this activation energy.
Heterogeneous catalysts are usually solids, powdered in order to maximize their surface area. Of particular importance in heterogeneous catalysis are theplatinum group metals and other transition metals, which are used inhydrogenations,catalytic reforming and in the synthesis of commodity chemicals such asnitric acid andammonia. Acids are an example of a homogeneous catalyst, they increase the nucleophilicity ofcarbonyls, allowing a reaction that would not otherwise proceed with electrophiles. The advantage of homogeneous catalysts is the ease of mixing them with the reactants, but they may also be difficult to separate from the products. Therefore, heterogeneous catalysts are preferred in many industrial processes.[43]
Inorganic chemistry, in addition to oxidation, reduction or acid-base reactions, a number of other reactions can take place which involvescovalent bonds between carbon atoms or carbon andheteroatoms (such as oxygen, nitrogen,halogens, etc.). Many specific reactions in organic chemistry arename reactions designated after their discoverers.
One of the most industrially important reactions is thecracking of heavyhydrocarbons atoil refineries to create smaller, simpler molecules. This process is used to manufacturegasoline. Specific types of organic reactions may be grouped by their reaction mechanisms (particularly substitution, addition and elimination) or by the types of products they produce (for example,methylation,polymerisation andhalogenation).
In the first type, anucleophile, an atom or molecule with an excess of electrons and thus a negative charge orpartial charge, replaces another atom or part of the "substrate" molecule. The electron pair from the nucleophile attacks the substrate forming a new bond, while theleaving group departs with an electron pair. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged. Examples of nucleophiles arehydroxide ion,alkoxides,amines andhalides. This type of reaction is found mainly inaliphatic hydrocarbons, and rarely inaromatic hydrocarbon. The latter have high electron density and enternucleophilic aromatic substitution only with very strongelectron withdrawing groups. Nucleophilic substitution can take place by two different mechanisms,SN1 andSN2. In their names, S stands for substitution, N for nucleophilic, and the number represents thekinetic order of the reaction, unimolecular or bimolecular.[45]
The three steps of anSN2 reaction. The nucleophile is green and the leaving group is red
The SN1 reaction proceeds in two steps. First, theleaving group is eliminated creating acarbocation. This is followed by a rapid reaction with the nucleophile.[46]
In the SN2 mechanisms, the nucleophile forms a transition state with the attacked molecule, and only then the leaving group is cleaved. These two mechanisms differ in thestereochemistry of the products. SN1 leads to the non-stereospecific addition and does not result in achiral center, but rather in a set ofgeometric isomers (cis/trans). In contrast, a reversal (Walden inversion) of the previously existing stereochemistry is observed in the SN2 mechanism.[47]
Electrophilic substitution is the counterpart of the nucleophilic substitution in that the attacking atom or molecule, anelectrophile, has low electron density and thus a positive charge. Typical electrophiles are the carbon atom ofcarbonyl groups, carbocations orsulfur ornitronium cations. This reaction takes place almost exclusively in aromatic hydrocarbons, where it is calledelectrophilic aromatic substitution. The electrophile attack results in the so-called σ-complex, a transition state in which the aromatic system is abolished. Then, the leaving group, usually a proton, is split off and the aromaticity is restored. An alternative to aromatic substitution is electrophilic aliphatic substitution. It is similar to the nucleophilic aliphatic substitution and also has two major types, SE1 and SE2.[48]
Mechanism of electrophilic aromatic substitution
In the third type of substitution reaction, radical substitution, the attacking particle is aradical.[44] This process usually takes the form of achain reaction, for example in the reaction of alkanes with halogens. In the first step, light or heat disintegrates the halogen-containing molecules producing radicals. Then the reaction proceeds as an avalanche until two radicals meet and recombine.[49]
Reactions during the chain reaction of radical substitution
Addition and elimination
Theaddition and its counterpart, theelimination, are reactions that change the number of substituents on the carbon atom, and form or cleavemultiple bonds.Double andtriple bonds can be produced by eliminating a suitable leaving group. Similar to the nucleophilic substitution, there are several possible reaction mechanisms that are named after the respective reaction order. In the E1 mechanism, the leaving group is ejected first, forming a carbocation. The next step, the formation of the double bond, takes place with the elimination of a proton (deprotonation). The leaving order is reversed in the E1cb mechanism, that is the proton is split off first. This mechanism requires the participation of a base.[50] Because of the similar conditions, both reactions in the E1 or E1cb elimination always compete with the SN1 substitution.[51]
E1 elimination
E1cb elimination
E2 elimination
The E2 mechanism also requires a base, but there the attack of the base and the elimination of the leaving group proceed simultaneously and produce no ionic intermediate. In contrast to the E1 eliminations, different stereochemical configurations are possible for the reaction product in the E2 mechanism, because the attack of the base preferentially occurs in the anti-position with respect to the leaving group. Because of the similar conditions and reagents, the E2 elimination is always in competition with the SN2-substitution.[52]
Electrophilic addition of hydrogen bromide
The counterpart of elimination is an addition where double or triple bonds are converted into single bonds. Similar to substitution reactions, there are several types of additions distinguished by the type of the attacking particle. For example, in theelectrophilic addition ofhydrogen bromide, an electrophile (proton) attacks the double bond forming acarbocation, which then reacts with the nucleophile (bromine). The carbocation can be formed on either side of the double bond depending on the groups attached to its ends, and the preferred configuration can be predicted with theMarkovnikov's rule.[53] This rule states that "In the heterolytic addition of a polar molecule to an alkene or alkyne, the more electronegative (nucleophilic) atom (or part) of the polar molecule becomes attached to the carbon atom bearing the smaller number of hydrogen atoms."[54]
If the addition of a functional group takes place at the less substituted carbon atom of the double bond, then the electrophilic substitution with acids is not possible. In this case, one has to use thehydroboration–oxidation reaction, wherein the first step, theboron atom acts as electrophile and adds to the less substituted carbon atom. In the second step, the nucleophilichydroperoxide or halogenanion attacks the boron atom.[55]
While the addition to the electron-rich alkenes and alkynes is mainly electrophilic, thenucleophilic addition plays an important role in the carbon-heteroatom multiple bonds, and especially its most important representative, the carbonyl group. This process is often associated with elimination so that after the reaction the carbonyl group is present again. It is, therefore, called an addition-elimination reaction and may occur in carboxylic acid derivatives such as chlorides, esters or anhydrides. This reaction is often catalyzed by acids or bases, where the acids increase the electrophilicity of the carbonyl group by binding to the oxygen atom, whereas the bases enhance the nucleophilicity of the attacking nucleophile.[56]
Some additions which can not be executed with nucleophiles and electrophiles can be succeeded with free radicals. As with the free-radical substitution, theradical addition proceeds as a chain reaction, and such reactions are the basis of thefree-radical polymerization.[60]
Cyclic rearrangements includecycloadditions and, more generally,pericyclic reactions, wherein two or more double bond-containing molecules form a cyclic molecule. An important example of cycloaddition reaction is theDiels–Alder reaction (the so-called [4+2] cycloaddition) between a conjugateddiene and a substitutedalkene to form a substitutedcyclohexene system.[62]
Whether a certain cycloaddition would proceed depends on the electronic orbitals of the participating species, as only orbitals with the same sign ofwave function will overlap and interact constructively to form new bonds. Cycloaddition is usually assisted by light or heat. These perturbations result in a different arrangement of electrons in the excited state of the involved molecules and therefore in different effects. For example, the [4+2] Diels-Alder reactions can be assisted by heat whereas the [2+2] cycloaddition is selectively induced by light.[63] Because of the orbital character, the potential for developingstereoisomeric products upon cycloaddition is limited, as described by theWoodward–Hoffmann rules.[64]
Biochemical reactions
Illustration of the induced fit model of enzyme activity
Biochemical reactions are mainly controlled by complexproteins calledenzymes, which are usually specialized tocatalyze only a single, specific reaction. The reaction takes place in theactive site, a small part of the enzyme which is usually found in a cleft or pocket lined byamino acid residues, and the rest of the enzyme is used mainly for stabilization. The catalytic action of enzymes relies on several mechanisms including the molecular shape ("induced fit"), bond strain, proximity and orientation of molecules relative to the enzyme, proton donation or withdrawal (acid/base catalysis), electrostatic interactions and many others.[65]
The biochemical reactions that occur in living organisms are collectively known asmetabolism. Among the most important of its mechanisms is theanabolism, in which differentDNA and enzyme-controlled processes result in the production of large molecules such asproteins andcarbohydrates from smaller units.[66]Bioenergetics studies the sources of energy for such reactions. Important energy sources areglucose andoxygen, which can be produced by plants viaphotosynthesis or assimilated from food and air, respectively. All organisms use this energy to produceadenosine triphosphate (ATP), which can then be used to energize other reactions. Decomposition of organic material byfungi,bacteria and othermicro-organisms is also within the scope ofbiochemistry.
Applications
Thermite reaction proceeding in railway welding. Shortly after this, the liquid iron flows into the mould around the rail gap.
Chemical reactions are central tochemical engineering, where they are used for the synthesis of new compounds from natural raw materials such aspetroleum, mineralores, andoxygen in air. It is essential to make the reaction as efficient as possible, maximizing the yield and minimizing the number of reagents, energy inputs and waste.Catalysts are especially helpful for reducing the energy required for the reaction and increasing itsreaction rate.[67][68]
Some specific reactions have their niche applications. For example, thethermite reaction is used to generate light and heat inpyrotechnics andwelding. Although it is less controllable than the more conventionaloxy-fuel welding,arc welding andflash welding, it requires much less equipment and is still used to mend rails, especially in remote areas.[69]
Monitoring
Mechanisms of monitoring chemical reactions depend strongly on the reaction rate. Relatively slow processes can be analyzed in situ for the concentrations and identities of the individual ingredients. Important tools of real-time analysis are the measurement ofpH and analysis of optical absorption (color) and emission spectra. A less accessible but rather efficient method is the introduction of a radioactive isotope into the reaction and monitoring how it changes over time and where it moves to; this method is often used to analyze the redistribution of substances in the human body. Faster reactions are usually studied withultrafast laser spectroscopy where utilization offemtosecondlasers allows short-lived transition states to be monitored at a time scaled down to a few femtoseconds.[70]
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^Latscha, Hans Peter; Kazmaier, Uli; Klein, Helmut Alfons (2008).Organische Chemie: Chemie-basiswissen II (in German). Vol. 2 (6th ed.).Springer. p. 273.ISBN978-3-540-77106-7.
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